The present invention relates to an optical disk apparatus which is arranged to optically reproduce a signal from a disk or record it thereon, and more particularly to the optical disk apparatus which enables to treat two or more kinds of disks whose recording planes have respective heights.
The commercially available optical disk apparatus is a compact disk (termed CD) player widely prevailed for reproducing music or a laser disk (termed LD) player. The compact disk and the laser disk have their own outer diameters and thicknesses but the same height, 1.2 mm, of the recording plane as each other. Hence, the common optical system may apply to both of the compact disk and the laser disk, so that the apparatus for reproducing both the CD and the LD has been merchandised. This sort of apparatus is "CLD-Z1" manufactured by Pioneer, Ltd, for example.
Today, there has been proposed a digital video disk (termed DVD) which has a higher recording density so that the DVD can record video data as well as audio data. For enhancing the recording density, the DVD needs a narrower track pitch, a shorter laser wavelength, and a greater numeral aperture (termed NA) of a lens than the CD. Concretely, for the DVD, the track pitch is 0.74 .mu.m, the laser wavelength is 650 nm, and the lens numeral aperture is 0.60, while for the CD, the track pitch is 1.6 .mu.m, the laser wavelength is 780 nm, and the NA of the lens is 0.4. The larger numeral aperture of the DVD results in disadvantageously reducing an allowable value of an angle shifted from the vertical of the disk plane against an optical axis of a light pick-up (tilt angle). In order to alleviate this disadvantage, if the DVD has the outer diameter of 120 mm which is the same as that of the CD, the height of the recording plane is 0.6 mm (the thickness of the two pasted disks is 1.2 mm) as compared with the height, 1.2 mm, of the CD (single disk). Hence, it looks like that the DVD disk is similar to the CD. In actual, however, both of the disks have respective standards. Apparently, the optical disk apparatus to be merchandised in near future is required to reproduce the data from the CD as well as the DVD.
Since the CD and the DVD have the different standards from each other, however, the optical disk apparatus is required to switch an optical system and a circuit system for the CD to those for the DVD or vice versa. This needs to discriminate which type of the disk is mounted to the apparatus when the disk is mounted thereto. That is, when a disk is mounted to the apparatus, the optical disk apparatus is requested to discriminate the type of the disk, the CD or the DVD, automatically switch the suitable optical and circuit systems transparently to the user, and properly record or reproduce the data on or from the mounted disk.
There is known JP-A-4-95224 disclosing a technique in that there are provided N (N.gtoreq.2) optical disks having different thickness with each other and N light collecting optical systems in corresponding to the N optical disks, and one light collecting optical system among them is suitably selected in accordance with a certain optical disk by utilizing an identification hole of a cartridge receiving the optical disk and exist or absence of a tracking error signal. However, JP-A-4-95224 is quite silent on a technique for focusing in order to obtain the tracking error signal. JP-A-8-339569 shows an optical disk system in which objective lenses and the degrees of amplitude of amplifier are exchanged in accordance with the kinds of optical disks in order to obtain a stable reproduction, however a specific measure for discriminating the kinds of optical disks is never disclosed. Moreover, JP-A-3-116442 shows a technique in that the kinds of optical disk are discriminated as the whether the read only type optical disk or the random access type optical disk by utilizing the reflection ratio of the reflected light beam from the optical disk. However, JP-A-3-116442 could not discriminate as to whether CD or DVD by merely using the reflection ratio.